Copper Phenoxyl Complexes

Abstract

The work described in the following was inspired by radical copper enzymes such as Galactose Oxidase (GO). GO catalyses the two-electron oxidation of terminal alcohols to the corresponding aldehydes using air oxygen. Transfer of two electrons is possible, since GO contains two magnetically coupled one-electron redox centres: a tyrosylate ligand from the apo-protein, which exists either in the tyrosylate or the tyrosyl radical form and is bound to a copper ion possessing two stable oxidation states (+I and +II). The catalytic activity of GO can be assigned to the Cu-OTyr (Tyr = Tyrosine, or more general Cu-Oaryl) motive, which is also found in all complexes synthesised and characterised in this thesis. The ligands specifically designed for this study, contain substituted, non-substituted or aromatically enlarged phenoxy moieties and belong to various compound classes: O,N,O pincer ligands, O,O',N donor ligands, salen type ligands, phenol-substituted triazole ligands, phenalenone ligands, benzoquinone ligands und acridine ligands. All of them were used to synthesise Cu(II) complexes, selected ligands (e.g. O,O',N, donor ligands) were additionally coordinated to Ni(II), Zn(II), Fe(II), Fe(III) and Co(II). All compounds were fully characterised using NMR or EPR spectroscopy, UV/vis/NIR-absorption spectroscopy, emission spectroscopy, cyclic voltammetry, spectroelectrochemistry, elemental analysis and XRD. These studies focus on (a) the electrochemical properties of the two one-electron redox couples Cu(II)/Cu(I) und [PhO�+]/[PhO], (b) the influence of ligand- and complex structure on both redox pairs and (c) the catalytic activity of the complexes resulting from their electrochemical properties. The latter was investigated by test reactions using benzyl alcohol as substrate and an in situ generated catalyst. Furthermore, detailed investigations on reactions yielding the active radical species Cu(II)-[OPh�+] under catalysis conditions were performed using a phenol-substituted triazole ligand system. Two methods were compared, one starting from a Cu(I) precursor, which is oxidised by air oxygen to yield the copper radical complex and the second starting from Cu(II) complexes which undergo a disproportionation reaction forming the active radical species and a Cu(I) byproduct.